The present invention relates to couplings for limiting the loading on the valve backseat and, in particular, a method of limiting backseat loading on an actuator operated valve stem to a predetermined loading.
A coupling for limiting the forces applied to an actuator operated valve stem during backseating is disclosed in the copending application U.S. Ser. No. 663,786 entitled VALVE-ACTUATOR COUPLING filed on Mar. 4, 1976 in the name of T. E. Kunkle and assigned to the Assignee of the present invention. Therein, in order to limit the force transmitted to the backseated valve stem under certain abnormal operating conditions, a limitedly deflectable coupling is provided at the operative connection between actuator and the stem to accommodate variable axial stop positions while limiting loading therebetween, both at the backseat stop position and during the valve stem movement.
The coupling includes internal Belleville springs disposed in a lost motion connection between limitedly axially movable fittings of the coupling. The arrangement is such that, during closing movement on the valve stem, a direct compressive force is applied. Upon initial unseating of the valve, the spring is increasingly deflected until fully compressed after which load direct tensile forces were applied to the stem. After unseating, the load is reduced during opening movement. When the stem arrives at the backseated position, the spring increasingly deflects until the actuator reaches its stop position. The stop positions on the valve stem and the actuator limit spring deflection when the members are at these positions to insure that the backseat loading is at substantially the predetermined level. This is accomplished by establishing predetermined spacial relationships of the coupling components based on the assumption that the stops are within a predetermined tolerance. This is sought to be achieved by interrelating the operative positions of totally internally captured parts, and not by actually providing an affirmative loading adjustment. The loading control is thus only within the rough limits provided by stack-up tolerances on the referenced parts.
The present invention, on the other hand, provides a coupling design which is presetable to a predetermined loading level and which can be field assembled between the valve and actuator in a manner which assures that when the valve is backseated the loading level is not exceeded. More particularly, the present coupling comprises first and second fittings operatively interconnected at a lost motion connection and respectively connected to the valve stem and the actuator. A Belleville spring assembly, operatively interposed in the lost motion connection, provides limited resilient deflection upon relative axial separation of the fittings. Load adjustment screws are provided on the fittings for deflecting the spring assembly an amount corresponding to a desired loading condition between the members. A loading interface is provided which is operator viewable through an access window. The loading level is such that when the valve stem is backseated, an interface separation then exists indicative that the level has been exceeded.
The coupling is operatively assembled by threadedly connecting the inner fitting to the actuator end with the spring assembly and an adjusting collar loosely received thereover. The outer fitting includes a bearing block telescoped over the actuator. The mounted components and the collar are threadedly connected until internal components are in axial contact so as to take only any axial clearances and provide a slight loading at the fully extended position. The outer fitting is then locked to the inner fitting. A plurality of axial adjusting screws are then adjusted to deflect the spring assembly until the predetermined loading level is established between the inner fitting and the bearing block. The interface between the inner fitting and the bearing block is observable through a window in the side of the outer fitting. The actuator is then lowered until the outer fitting engages the threaded end of the valve stem. The outer coupling is then threaded slightly onto this threaded end and the actuator is retracted to its upper stop position. The extension provided by the coupling at this point is such that the valve end is spaced from the backseat and the springs are not deflected. The outer fitting is then rotated to increasingly thread the coupling onto the valve end until first, the valve is backseated and second, upon further threading, the interface separates indicative that the loading level has been reached. The fitting is then locked onto the valve end such that the inner and the outer fittings are fixedly connected to their respective actuating members. The adjusting screws are then removed. Thus, each time the valve reaches the backseated position, the load on the stem will be limited to a predetermined level. In the event of actuator failure, inasmuch as the actuator end is fixed, the force transmitted thereto is borne by the actuator assembly without any load increase on the valve stem. Thus, with the above coupling and method of assembly, the valve backseat loading can be readily and accurately achieved and thereafter limited.